Diodes find use in many solid-state devices. Diodes which respond to light, known as photodiodes, are widely used in many solid-state image sensors, also known as imagers, which were developed in the late 1960s and early 1970s primarily for video image acquisition, transmission, and display. An imager employing a photosensitive diode absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation. Typical devices which use photosensitive diodes include charge coupled devices (CCDs), photodiode arrays, charge injection devices, hybrid focal plan arrays, memory and logic circuits and CMOS imagers.
In the case of photosensitive devices such as CCD's or CMOS imagers, the photosensitive region is typically a p-n junction. This junction is reverse biased by applying an electrical potential Vo which reverse biases the p-n junction. The p-n junction is then isolated typically by turning off a “reset transistor” that is used to reverse bias the junction. Under illumination, incident light photons create electron-hole pairs that are separated by the electric field in a depletion region of the p-n junction. This separation by the electric field results in one charge type being stored in the depletion region, thus collapsing the depletion region, and reducing the voltage Vo across the p-n junction. It is this reduction in voltage across the diode p-n junction due to light exposure that is measured.
To provide context for the invention, an exemplary prior art photodiode is described below with reference to FIG. 1. However, it is to be understood that the invention also has utility in any semiconductor diode device which includes a p-n junction where there is leakage to the substrate. Accordingly, the present invention may also find utility in semiconductor devices where p-n junctions are shielded from light, where p-n junctions are formed in a p-well, where p-n junctions are formed in an n-well, or more complicated junctions such, for example, a p-n-p junction. Also, while FIG. 1 shows a simplified photodiode 15 for use as a pixel of an imager, it should be understood that the single photodiode pixel 15 in practical use will be a part of either a row of pixels or an M×N array of pixels arranged in rows and columns.
The photodiode 15 of FIG. 1 is shown in part as a cross-sectional view of a semiconductor substrate 10 doped with a p-type material to form p-well 12. A field oxide region 20, which serves to surround and isolate the photodiode 15 may be formed by thermal oxidation of the doped substrate 10, or by chemical vapor deposition of an oxide material as in the STI (shallow trench isolation) process. More highly doped p-type regions 40 are formed under the field oxide region 20 and an n-type implant 30 is formed between the field oxide regions 20. The field oxide regions 20 may be formed before or after doped regions 30, 40. As shown in FIG. 1, p-type regions 40 and n-type implant 30 are typically doped so as to form a junction of regions 30 and 40 that is aligned or self-aligned to the edge of the field oxide 20. However, the overlapping of the p-type region 40 and the n-type region 30 results in current leakage from the photodiode to the substrate 10 through the depletion region.
The resolution of the imaging device is a function of the size and performance of each photodetector. To improve resolution, such as by presenting more image lines per inch of visual display, a greater number of photoconductors are required per unit area of the photodetector array. Imager performance is degraded if individual photodiodes have high charge leakage. It is important that the photodetector array be fabricated to allow the photodiode charge to be accurately read within the allowed data sampling time of the system.
There is needed, therefore, an improved photodiode for use in an imager apparatus that exhibits decreased charge leakage to the substrate through the depletion region. There is also need for an improved p-n junction diode for other integrated circuit applications, such as DRAM applications, which likewise exhibits reduced charge leakage to the substrate through a depletion region. A method of fabricating diodes, including photodiodes exhibiting these improvements is also needed.